In contactless chip cards, there are known ways of transmitting binary data from the user device to the chip cards by means of a signal transmitted at a carrier frequency f (i.e., 13.56 MHz) that is amplitude modulated by binary digits of a code to be transmitted. Contact chip cards work accurately only if they are able, at very high speed and with certainty, to recognize that the user device is communicating by means of radio frequency signals so the chip cards can place themselves in a mode of operation suited to recognizing signals of this type.
Recognizing radio frequency signals in the prior art is done by means of an electronic device of the type described in FIG. 1. This device comprises a circuit 10 for the detection of radio frequency signals which includes a resonant circuit comprising an antenna loop taking the form of a winding 20 and a capacitor C.sub.0 which are parallel-connected. Circuit 10 provides a signal V.sub.AC whose waveform is shown in FIG. 2a.
The device further comprises a rectifier and filter circuit 12 which includes a four-diode bridge 22 coupled to a capacitor C.sub.1 to provide a full-wave rectified signal V.sub.DC whose waveform is shown in FIG. 2b. Also included in the rectifier and filter circuit 12 is a resistor 14 and a comparator circuit 16 for comparing the potentials of V.sub.DC and V.sub.DD at the terminals of the resistor 14. The comparator circuit 16 provides a signal V.sub.RF at its output indicating the presence of binary amplitude-modulated radio frequency signals. This signal V.sub.RF. informs the microcontroller 50 that the signal transmitted to the card is of the radio frequency type.
The invention is aimed at obtaining a detector of radio frequency signals for a chip card for determining the presence of radio frequency signals at high speed and with certainty. To this end, the detector, according to the invention, uses the signals at the carrier frequency.